Rice rhizospheric effects and mechanism on soil cadmium bioavailability during silicon application.

Exogenous Si mitigates the mobility and bioavailability of Cd in the soil, thereby alleviating its phytotoxicity. This study focused on specific Si-induced immobilisation effects within the rhizosphere (S1), near-rhizosphere (S2), and far-rhizosphere (S3) zones. Based on the rhizobox experiment, we found that applying Si significantly elevated soil pH, and the variation amplitudes in the S3 soil exceeded those in the S1 and S2 soils. Si-induced changes in the rhizosphere also included enhanced dissolved organic carbon and diminished soil Eh, particularly in the Si400 treatment. Meanwhile, the introduction of Si greatly enhanced the Fe2+ and Mn2+ concentrations in the S1 soil, but reduced them in the S2 soil. The rhizosphere effect of Si which enriched Fe2+ and Mn2+ subsequently promoted the formation of Fe and Mn oxides/hydro-oxides near the rice roots. Consequently, the addition of Si significantly reduced the available Cd concentrations in S1, surpassing the reductions in S2 and S3. Moreover, Si-treated rice exhibited increased Fe plaque generation and fixation on soil Cd, resulting in decreased Cd concentrations in rice tissues, accompanied by reduced Cd translocation from roots to shoots and shoots to grains. Structural equation modelling further highlighted that Si is essential in Cd availability in S1 and Fe plaque development, ultimately mitigating Cd accumulation in rice. Si-treated rice also exhibited higher biomass and grain yield than those of control groups. These findings provide valuable insights into Si-based strategies for addressing the Cd contamination of agricultural soils.

Metagenomic and proteomic insights into the self-adaptive cell surface hydrophobicity of Sphingomonas sp. strain PAH02 reducing the migration of cadmium-phenanthrene co-pollutant in rice.

Cell surface hydrophobicity (CSH) dominates the interactions between rhizobacteria and pollutants at the soil-water interface, which is critical for understanding the dissipation of pollutants in the rhizosphere microzone of rice. Herein, we explored the effects of self-adaptive CSH of Sphingomonas sp. strain PAH02 on the translocation and biotransformation behaviour of cadmium-phenanthrene (Cd-Phe) co-pollutant in rice and rhizosphere microbiome. We evidenced that strain PAH02 reduced the adsorption of Cd-Phe co-pollutant on the rice root surface while enhancing the degradation of Phe and adsorption of Cd via its self-adaptive CSH in the hydroponic experiment. The significant upregulation of key protein expression levels such as MerR, ARHDs and enoyl-CoA hydratase/isomerase, ensures self-adaptive CSH to cope with the stress of Cd-Phe co-pollutant. Consistently, the bioaugmentation of strain PAH02 promoted the formation of core microbiota in the rhizosphere soil of rice (Oryza sativa L.), such as Bradyrhizobium and Streptomyces and induced gene enrichment of CusA and PobA that are strongly associated with pollutant transformation. Consequently, the contents of Cd and Phe in rice grains at maturity decreased by 17.2% ± 0.2% and 65.7% ± 0.3%, respectively, after the bioaugmentation of strain PAH02. These findings present new opportunities for the implementation of rhizosphere bioremediation strategies of co-contaminants in paddy fields.

Influence of landform, soil properties, soil Cd pollution and rainfall on the spatial variation of Cd in rice: Contribution and pathway models based on big data.

Landform, soil properties, soil cadmium (Cd) pollution and rainfall are the important factors affecting the spatial variation of rice Cd. In this study, we conducted big data mining and model analysis of 150,000 rice-soil sampling sites to examine the effects by the above four factors on the spatial variation of rice Cd in Hunan Province, China. Specifically, the variable coefficient of rice Cd in space was significantly correlated with the partition scale according to the logistic fitting. The improved random forest results suggested that elevation (DEM) and pH were the two most important factors affecting the spatial variation of rice Cd, followed by relief, soil Cd content and rainfall. Typically, variance partitioning analysis (VPA) revealed that both the soil property and the interactive effects between the soil property and Cd pollution were the principal contributors to the rice-Cd variation, with the respective contributing rates of 30.5 % and 29.0 %. Meanwhile, the partial least square-structural equation modelling (PLS-SEM) elucidated 4 main paths of specific indirect effects on rice-Cd variation. They were landform → physicochemical property → soil acidity → rice-Cd variation, landform → soil acidity → rice-Cd variation, physicochemical property → soil acidity → rice-Cd variation, and soil texture → soil acidity → rice-Cd variation. This work can provide a general guidance for scientific zoning, accurate prediction and prevention of Cd pollution in paddy fields.

[Effects of Equal Amounts Silicon Fertilizer Application on Soil Bioavailability of Cadmium and Cadmium Uptake by Rice].

Cadmium (Cd) contamination of paddy fields is a global concern, as it can cause the accumulation of Cd in food. To explore the effects of equal application of silicon fertilizers on the bioavailability of cadmium and soil Cd uptake at different growth stages of rice, a field experiment was conducted with five silicon fertilizers under the same silicon dose (225 kg·hm-2). The results revealed that the Cd contents in roots, stems, and leaves increased with the extension of the rice growth stage. The application of silicon fertilizers reduced the Cd contents in roots, stems, and leaves in brown rice by 14.9%, 28.2%, and 12.2%, respectively. Compared with that in the control, the Cd content of brown rice in the SiCaMgFe and SiW treatments was decreased by 21.1% (P<0.05) and 21.2% (P<0.05), respectively. Similarly, Cd content in iron plaque (DCB-Cd) increased with the extension of the rice growth period, which accounted for 15.8%-42.8% of the total Cd content in roots, and the DCB-Cd content was different in each stage of rice. The content of exchangeable Cd (Exc-Cd) in soil at the mature stage of rice decreased by 36.4%, and the other fractions increased by 12.5%-48.2%. The results showed significant negative correlations between the Cd contents and Si in roots, DCB-Cd and soil available Cd and available Si, Exc-Cd and Car-Cd, and soil available Cd and pH value. Cd content in roots was positively correlated with DCB-Cd. With the equal dose of silicon fertilizer, the treatments of SiCaMgFe and SiW could effectively reduce the Cd content in rice. The application of silicon fertilizer promoted the transfer of Exc-Cd to Carb-Cd by increasing the soil pH value and the soil available Si content, meanwhile reducing the soil available Cd, Exc-Cd contents, the adsorption of Cd by the iron film on the root surface, and the adsorption capacity of iron plaque and root, thereby reducing the absorption of Cd by rice.

Enhanced As extraction from paddy soils with high As contamination risk by rice plant upon Si fertilization.

Owing to flooded growing conditions and specific physiological characteristics, rice plant is more efficient in As uptake and accumulation, which provides a cost-effective and time-efficient pathway to deplete bioavailable As from paddy soils. In the present study, the enhancing effect of silicon (Si) fertilization on As extraction from heavily contaminated paddy soils by rice was explored Upon incorporation of one weak acid Si fertilizer (AcSF), soil As solubility was significantly promoted by 1.3-1.4-fold, while a slightly increase in porewater As was observed with alkaline soluble Si fertilizer Na2SiO3 (AlSF). With both Si fertilizers applied before transplanting, a relatively low Si/As molar ratio (<100) in soil porewater was obtained, As a result, soil As uptake by rice plant with Si fertilizers was enhanced by 37.2%-171.7% compared to control (CK). Notably, up to 91.6% of the total As in rice plant retained in root with Si fertilization, suggesting the importance of root removal. By harvesting the whole rice plant including roots, soil bioavailable As measured by diffusive gradients in thin films (DGT) declined by 26.9%-31.3% in AlSF treatments relative to CK. Total soil As depletion by the whole rice plant was significantly enhanced from 2.8% in CK to 7.0%-11.2% in Si fertilizer treatments. In this way, 197.5 mg As m-2-232.5 mg As m-2 could be eliminated from soil following one rice-growth season, which was 2.3-2.7-fold higher compared to CK. These results identified the effectiveness of soluble Si fertilizer in enhancing soil As depletion by rice from paddy soils with high As contamination risk, which could serve as a cost-effective strategy with little technical-restriction.

Silicon facilitated the physical barrier and adsorption of cadmium of iron plaque by changing the biochemical composition to reduce cadmium absorption of rice roots.

Silicon effectively inhibits cadmium (Cd) uptake in rice, iron plaque on root surface was the primary link and first interface of Cd entering into rice root. To elucidate the mechanism of iron plaque under silicon treatment on root Cd uptake, the morphological characteristics of iron plaque, mechanisms of Cd adsorption of iron plaque and effect of iron plaque on Cd uptake by rice roots of Yuzhenxiang (YZX) and Xiangwanxian (XWX) rice varieties were studied by employing energy spectrum analysis technique, non-invasive micro-test technique, and isothermal-kinetic adsorption method. Scanning electron microscopy-X-ray energy dispersive (SEM-EDX) analysis showed that denser crystal structure of iron plaque was observed at Si treatment, silicon promoted the thickening of iron plaque and strengthened the isolation of iron plaque to Cd, which reduced the Cd content of white roots of YZX and XWX varieties by 30.2% and 20.9% respectively. However, the blocking effect of iron plaque on Cd was weakened under silicon treatment with iron plaque removed, Cd content in iron plaque of YZX and XWX cultivars was significantly decreased by 36.3% and 18.4%, Cd concentrations in white root and shoot was significantly increased, and the influxes of Cd2+ at elongation and maturation zone of root were increased in multiples. The results of adsorption test showed that the adsorption process of iron plaque was mainly a monolayer adsorption completed by boundary diffusion. The X-ray photoelectron spectroscopy (XPS) results demonstrated that silicon changed the biochemical composition of iron plaque and increased the density of the carbon-oxygen bound groups on iron plaque, which is the most likely reasons for the higher affinity of Cd adsorption ability of iron plaque observed in the silicon treated iron plaque. This study suggested the silicon-facilitated iron plaque have played critical effects in controlling the Cd accumulation in rice roots by changing the morphology and chemical composition of iron plaque.

Remediation of Cd contaminated paddy fields by intercropping of the high- and low- Cd-accumulating rice cultivars.

Intercropping cadmium (Cd) hyperaccumulators with crops have been widely applied in the remediation of contaminated farmland soils. However, most studies were done on drylands since the majority of the hyperaccumulators are susceptible to the aquatic environment, making the remediation of Cd-contaminated paddy fields particularly difficult. Our study attempts to address the issue by intercropping the high-Cd-accumulating (henceforth, "high-Cd") rice cultivars with the low-Cd-accumulating (henceforth, "low-Cd") ones, and to study the Cd removal, uptake and translocation during the remediation process. The results indicated that intercropping mode with 20-cm row spacing (intercropping-20 treatment) performed better than the that with 30-cm row spacing (intercropping-30 treatment), while intercropping had stronger impact on late rice compared to early rice. In general, the physiological condition of rice was stable under the intercropping-20 treatment, suggesting the growth of rice was not impeded. For late rice, as the intercropping-20 treatment can significantly reduce soil pH and increase the diethylenetriaminepentaacetic acid extracted Cd (DTPA-extracted Cd) from the rhizosphere soil, Cd accumulated more in the tissues of the high-Cd rice cultivars (H2), and its dry biomass increased. As a result, a drastic improvement in the total Cd removal rate by 38.55 % was noticed. Therefore, the reduction of total Cd concentration in 0-20 cm profile caused by removal, thus it could provide safer soil environment for the growth of low Cd-rice cultivars (L2), leading to a significant drop in the root Cd concentration and safer production of L2. Interestingly, intercropping had no effect on the yield per plant of low-Cd rice cultivars. For early rice, intercropping-20 treatment exerted trivial effects to all aspects. The intercropping-30 treatment has poor representativeness of all indicators because of the large intercropping distance. Our results demonstrate that intercropping of the high-Cd and the low-Cd rice cultivars is a potential mode for Cd remediation in paddy fields.

[Primary Factors Affecting Soil Bioavailable Cadmium and Asenic by Different Properties of Silicon Fertilizer].

To clarify the primary factors affecting soil bioavailable cadmium (Cd) and arsenic (As) by silicon fertilizer, we chose different properties of silicon fertilizer, including three types of alkaline silicon fertilizer[Na2SiO3, CaSiO3, and ASSF (pH 9-11)] and one weak acid neutral soluble silicon fertilizer (NSSF, pH 5-6), to carry out a pot experiment by adding different amounts of Si fertilizer (25-800 mg·kg-1, calculated as Si). After 21 days of flooding, soil basic physical and chemical properties, along with diffusive gradients in thin film Cd and As (DGT-Cd and DGT-As) were investigated. The results showed that the application of Si fertilizer with different properties had different significant effects on the basic physical and chemical properties of soil. Specifically, the three types of alkaline silicon fertilizer significantly increased the soil pH (P<0.05), among which Na2SiO3 exhibited the strongest ability; however, the application of NSSF remarkably reduced soil pH (P<0.05), and per unit (mg) Si application of NSSF could reduce soil pH by 0.0017 units. Furthermore, with each fertilizer application rate that reached 400 mg·kg-1 (calculated as Si), the changes in soil pH and Eh tended to be gentle. The ability of the four types of silicon fertilizer to improve soil available silicon ranked as NSSF>Na2SiO3>ASSF>CaSiO3. Additionally, the application of the three types of alkaline silicon fertilizer apparently decreased soil DGT-Cd while increasing soil DGT-As (P<0.05). When the addition rate of CaSiO3 was up to 100 mg·kg-1(calculated as Si), soil DGT-Cd concentration could be significantly decreased by approximately 50.89% without causing a significant increase in soil DGT-As concentration. Conversely, when the NSSF application rate was up to 400 mg·kg-1 (calculated as Si), the soil DGT-As basically reached its steady-state, and the DGT-As reduction rate reached 85.87%. Strikingly, the correlation analysis of the influencing factors of soil DGT-Cd and DGT-As showed that soil pH was the main factor affecting soil bioavailable Cd and As (DGT-Cd and DGT-As), and the effect of soil available Si and P on soil Cd and As bioavailability was negligible. Consequently, soil DGT-Cd and soil DGT-As could reach a minimum when soil pH was adjusted to 6.5-7.0 or 5-5.5 by alkaline silicon fertilizer or NSSF, respectively. It is undoubtedly of great significance, to clarify the primary factors that influence soil bioavailable Cd and As to ensure food security production.

Alleviation of Cd-polluted paddy soils through Si fertilizer application and its effects on the soil microbial community.

In this study, the effects of slag-based Si fertilizers on Cd-polluted paddy soils, soil microbial diversity, and functional properties were evaluated through a long-term field experiment conducted in a double-rice cropping system in southern China. The results showed that soil pH significantly increased from 5.15 to 6.13 after seven years of Si fertilization. Cd accumulation in both the soil and rice plants were significantly decreased for all the Si fertilizers treatments. Treatments using Si fertilizer in powder form exhibited the best alleviation effects, where soil available Cd decreased from 0.50 mg kg-1 to 0.43 mg kg-1, and Cd accumulation in rice roots, straw, and grains decreased by 32.2 %, 57.2 %, and 45.5 %, respectively, than that in the control. Following Si application, the soil microbial richness and Shannon diversity increased from 6731 to 7549 and 7.12 to 7.28, respectively. Proteobacteria, Nitrospirae, and Gemmatimonadetes, were significantly enriched in the Si-treated samples, whereas Verrucomicrobia, Chlamydiia, Ktedonobacteria and Candidatus_Saccharibacteria exhibited opposite patterns. Bioinformatics analysis using phylogenetic investigation of communities by reconstruction of unobserved states tools revealed that the varied microbial community induced functional adaption of soil microorganisms involved in metabolism, genetic information processing, cellular processes, and environmental information processing. The soil pH, NH4-N, and available Cd and Si contents were the key factors that best explained the variations in bacterial community composition among different treatments. Slag-based Si fertilizers are effective for Cd detoxication and can benefit the growth of rice plants throng the regulation of soil microorganisms.

[Impacts of Uptake and Accumulation of Cd on Double Rice-Paddy Soil by Silicon Fertilizer Continuous Application].

The impacts of silicon (Si) fertilizers on cadmium (Cd) bioavailability in soil and Cd accumulation in paddy-rice plants were investigated in a field positioning test. The results indicated that the continuous application of 180 kg·hm-2 Si fertilizers improved soil bioavailable Si in paddy-rice fields by 108.1%-275.0% and improved pH values by 1.15-1.33 in soil. The difference in the contents of DTPA-Cd in soil by 12.3% and 15.9% was significant in early and late rice, respectively. The continuous application of silicon promoted the transformation of soil cadmium to a stable form and reduced the contents of exchangeable Cd and carbonate binding state Cd in the soil by 2.6%-5.1% and 8.6%-24.9%. The contents of ferric manganese oxide binding state Cd were increased significantly; meanwhile, the contents of organic binding state Cd and residue state Cd in soil were improved by 2.3%-12.8% and 2.3%-6.0%, respectively. The contents of Cd in the root, shoot, and rice were reduced by 38.4%, 49.7%, and 50.9% in early season rice and by 30.6%, 34.4%, and 39.2% in late season rice, respectively. The absorption factors and translocation factors were reduced significantly by 25.5%-49.6% and 13.5%-52.6%, and the average bioaccumulation factors were decreased by 6.0% and 8.0% in early and late season rice. The powder Si fertilizer had the best results overall. In conclusion, Si fertilizer could reduce the absorption and accumulation in rice; however, the continuous Cd reduction effect of continuous application Si fertilizer was not obvious.

Influence of soil properties on cadmium accumulation in vegetables: Thresholds, prediction and pathway models based on big data.

Soil properties, such as soil pH, soil organic matter (SOM), cation exchange capacity (CEC), are the most important factors affecting cadmium (Cd) accumulation in vegetables. In this study, we conducted big data mining of 31,342 soil and vegetable samples to examine the influence of soil properties (soil pH, SOM, CEC, Zn and Mn content) on the accumulation of Cd in root, solanaceous, and leafy vegetables in Hunan Province, China. Specifically, the Cd accumulation capability was in the following order: leafy vegetables > root vegetables > solanaceous vegetables. The soil property thresholds for safety production in vegetables were determined by establishing nonlinear models between Cd bioaccumulation factor (BCF) and the individual soil property, and were 6.5 (pH), 30.0 g/kg (SOM), 13.0 cmol/kg (CEC), 100-140 mg/kg (Zn), and 300-400 mg/kg (Mn). When soil property values were higher than the thresholds, Cd accumulation in vegetables tended to be stable. Prediction models showed that pH and soil Zn were the leading factors influencing Cd accumulation in root vegetables, explaining 87% of the variance; pH, SOM, soil Zn and Mn explained 68% of the variance in solanaceous vegetables; pH and SOM were the main contributors in leafy vegetables, explaining 65% of the variance. Further, variance partitioning analysis (VPA) revealed that the interaction effect of the corresponding key soil properties contributed mostly to BCF. Meanwhile, partial least squares (PLS) path modeling was employed to analyze the path and the interactive effects of soil properties on Cd BCF. pH and SOM were found to be the biggest two players affecting BCF in PLS-models, and the most substantial interactive influence paths of soil properties on BCF were different among the three types of vegetables.

Combined amendment improves soil health and Brown rice quality in paddy soils moderately and highly Co-contaminated with Cd and As.

In situ remediation technology applied aims to not only decrease cadmium (Cd) and arsenic (As) uptake by rice but also improve soil health and rice quality in contaminated paddy soils. Here the effects of a combined amendment, consisting of limestone, iron powder, silicon fertilizer, and calcium-magnesium-phosphate fertilizer, with three application rates (0, 450, and 900 g m-2) on soil health, rice root system, and brown rice quality were compared in moderately versus highly Cd and As co-contaminated paddy fields. After the amendment application, soil pH, cation exchange capacity, four kinds of soil enzyme activities increased (sucrase, urease, acid phosphatase, and catalase), and concentrations of leached Cd/As decreased, as measured by the DTPA (diethylene triamine pentaacetic acid) and TCLP (toxicity characteristic leaching procedure). Changes in the above soil indicators promoted soil health. In both fields, the dithionite-citrate-bicarbonate (DCB)-Fe and DCB-Mn concentration in iron plaque increased and root length became longer. Changes in the above root system indicators reduced the root system's absorption of Cd and As but increased that of nutrients. Under 900 g m-2 treatment, the Cd concentration in brown rice of two sites decreased by 55.8% and 28.9%, likewise inorganic As (iAs) decreased by 50.0% and 21.1%, whereas essential amino acids increased by 20.4% and 20.0%, respectively. Furthermore, the Cd and iAs concentrations in brown rice were <0.2 mg kg-1 (maximum contaminant level of Cd and iAs in the Chinese National Food Safety Standards GB2762-2017 for brown rice) under the 900 g m-2 in the moderately contaminated field. These results suggest the combined amendment can improve soil health and brown rice quality in the moderately and highly Cd- and As-co-contaminated paddy soils, offering potential eco-friendly and efficient remediation material for applications in such polluted paddy soils.

The role of silicon in cadmium alleviation by rice root cell wall retention and vacuole compartmentalization under different durations of Cd exposure.

Silicon (Si) plays a pivotal role in mitigating phytotoxicity caused by cadmium (Cd). However, few former reports focused on the internal mechanism how Si assisted in alleviating Cd stress in rice under different durations of Cd exposure. Herein, the effects of Si on subcellular distribution of Cd in rice roots under short-term (12 h) and long-term (20 d) Cd exposure were explored. Results showed that Si decreased shoot Cd concentration but had little impact on root Cd levels. Under short-term Cd exposure, subcellular distribution analysis showed that Si increased the ratio of Cd in root cell wall by 23.2~24.0%, and decreased the ratio of Cd in root soluble fraction by 20.6~21.5%. This suggested that Si supply improved root retention of Cd by fixing it on the cell wall and thus restricted intracellular transportation of Cd. Further analysis unraveled that pectin (especially ionic-soluble pectin) of the cell wall was the main binding component, and Si supply induced more Cd accumulation in covalent-soluble pectin and hemicellulose. Moreover, the overexpression of germin-like proteins (GLPs) proved the role of cell wall in moderating Cd toxicity. Under long-term Cd exposure, Si promoted phytochelatin 2 (PC2) and phytochelatin 3 (PC3) synthesis in cytosol, at the same time, Si down-regulated the expression of the Cd efflux-related protein multidrug resistance-associated protein-like ATP-binding cassette transporters (MRP-like ABC transporters) and limited Cd transportation from vacuole to cytosol. Taken together, Si rather predominates in limiting Cd translocation by the cell wall of root under short-term Cd exposure and promoting vacuole compartmentalization to mitigate the Cd toxicity under long-term exposure, instead of reducing the absorption of Cd in rice roots, thereby decreasing Cd delivery into shoots.

Depletion of bioavailable As/Cd with rice plant from paddy soils of high contamination risk.

Co-uptake and high accumulation of As and Cd by rice is an outstanding issue threatening public health. From the viewpoint of soil cleanup, however, efficient As/Cd extraction by this paddy-adapted plant, followed by biomass removal, could provide a major pathway depleting As/Cd accumulation in paddy soils and thus inhibiting their transfer in food chain. Here a field trial was performed to identify the significance of As/Cd cleanup from paddy soil by rice. 88 % and 51 % of total extracted As and Cd were retained in root. To eliminate specifically rice-available As/Cd pool and obstruct their cycling back to soil, one crop of rice root was removed, leading to the depletion of 46 % and 69 % of plant available As (soluble & exchangeable) and Cd (exchangeable & carbonate-bound), respectively. In the following cultivation on the post-cleanup field, polished rice As fell from 0.23 mg kg-1 to 0.12 mg kg-1, markedly lower than the Chinese (WHO) limit (0.2 mg kg-1). Meanwhile, white rice Cd decreased by 24 %. This field work identified that As/Cd co-extraction by paddy-adapted rice plant, followed by root removal, as a primary step toward rice safety in areas with high contamination risk but little reserved paddy resources.

[Temporal and Spatial Variation Patterns of Picophytoplankton and Their Correlations with Environmental Factors During the Wet Season in East Lake Dongting].

Picophytoplankton (<3 µm), comprising picocyanobacteria (PCY) and photosynthetic picoeukaryotes (PPEs), are considerably important in the material circulation and energy flow of aquatic ecosystems. To explore the temporal and spatial variation patterns of picophytoplankton and their correlations with environmental factors in lotic Yangtze-connected lakes, field in-situ investigations were performed on a monthly basis during the wet season (May to August) in 2019 in East Lake Dongting, a Yangtze-connected lake. The results indicated that both the Chla biomass and abundances of picophytoplankton exhibited significant spatial and temporal variability (P<0.05). The picophytoplankton Chla biomass showed an average concentration of 8.52 µg·L-1 and accounted for 41.6% to total phytoplankton on an average. From May to August, Chla biomass of picophytoplankton kept increasing with increasing temperature, especially in the north and south of the lake, and it was the lowest in the east of the lake. PCY dominated picophytoplankton abundance in East Lake Dongting and was 3.4 times the abundance of PPEs on an average. Similar spatial and temporal variation patterns were observed between PCY and PPEs. The abundances of PCY and PPEs both increased first and then decreased during the wet season. Spatially, picophytoplankton showed a trend to migrate from the northern lake to the southern lake from May to July, and the abundance significantly declined in August and peaked mainly in the north of the lake. The analysis results showed that picophytoplankton in East Lake Dongting exhibited significant spatial and temporal variability during the wet season; the water level and N：P ratio were determined to be the most important factors explaining the variation of the abundance proportion of PCY and PPEs.

Input and output of cadmium (Cd) for paddy soil in central south China: fluxes, mass balance, and model predictions.

It is important to provide a more comprehensive understanding of cadmium (Cd) input and output in different contamination zones. In this study, we choose 15 sampling areas in three types of contamination zones (industrial and mining, suburb, and rural) to systematically study the inventory of soil Cd input and output in Changzhutan (CZT) urban agglomerations, Hunan Province, China. The results showed that the value of total Cd input in industrial and mining (34.58 g/ha/year) was respectively about 2 and 3 times of that in suburb and in rural. Meanwhile, the total output flux in industrial and mining also presented highest value (38.67 g/ha/year) among the zones. As for the contributions, atmospheric deposition was responsible for 85-89% of the total input fluxes, which was significantly higher than those of irrigation water and fertilizer. Crop harvesting, especially straw removal, was the dominant output pathway, contributing 66-78%. Moreover, Cd annual balance illustrated that the net input fluxes under straw removal scenario were negative in all zones, and it was opposite under straw returning scenario. Further, the changes of soil Cd concentrations under straw returning and straw removal scenario were compared by a dynamic mathematical model. The modeling results presented that the soil Cd content continued to increase under straw returning in 100 years, while it was declining under straw removal scenario. This prediction indicated straw removal was an important remediation for Cd-polluted paddy soil, especially in Hunan. Nevertheless, more treatment measures need to conduct to reach the safety limits in paddy soil.

Biochar-assisted phytoextraction of arsenic in soil using Pteris vittata L.

The alkaline nature of biochar provides a potential for soil arsenic (As) mobilization and, hence, enhancing efficiency of As phytoextraction by combining with As hyperaccumulator. To testify the feasibility and potential risk of the above strategy, biochar effect on As transfer in a paddy soil and accumulation in P. vittata was investigated in a pot experiment. By leaching soil (total As concentration 141.17 mg/kg) with simulated acid rain (pH 4.2), As the concentration in leaching eluate increased proportionally with increasing biochar ratio. Coincident with elevated soil As mobility, apparent enhancement in As uptake and translocation in P. vittata was determined with 1-5% biochar amendment after 40 days of plant growth. Furthermore, diffusive gradients in thin film (DGT) technique were employed to characterize any potential risk in vertical downward migration of As at 2-mm resolution. A significantly increasing profile of DGT-As ranging from on average 20 µg/L in CK to 50-100 µg/L in 1-3% biochar treatments was recorded over 0-60 mm depth, with 25-71% lower labile As in the rhizosphere than non-rhizosphere zone with few exceptions. As compared to Chinese quality standard for groundwater (Class IV 50 µg/L), biochar ratio at ≤ 1% was suggested for local water safety while actual application should take the physicochemical characteristic of tested soil into account. Our results demonstrated the biochar-assisted P. vittata phytoremediation can serve as an emerging pathway to enhance efficiency of soil As phytoextraction. The combination of DGT techniques and greenhouse assay provided a powerful tool for evaluating the gradient distribution of heavy metal in rhizosphere and accessing corresponding ecological risk at more precise scale.

Effect of a novel Ca-Si composite mineral on Cd bioavailability, transport and accumulation in paddy soil-rice system.

Ubiquitous cadmium (Cd) contamination in mine impacted paddy soil has been jeopardizing regional rice quality, which represents a dominant pathway of Cd exposure in populations depending on a rice diet. Two major aspects of mitigation, soil liming and Si fertilization, were integrated and investigated with a Ca-Si-rich composite mineral (CS) derived from feldspar and carbonate. With the CS amendment, bioavailable Cd in rice rhizosphere was reduced by 92-100% from tillering to maturation stage, paralleled by a marked increase in Cd bound to Fe/Mn oxides and carbonate. As indicated by XRD analysis, the much reduced labile pool of Cd in the CS-amended soil could be mainly attributed to Cd (co)precipitation (Cd(OH)2, Cd2(OH)3Cl, CH6Br3CdN) and surface complexation on more negatively charged oxides at elevated soil pH with CS addition. EDX line scan illustrated much more intensified Si deposition along root cross-section in the CS treatment, which resulted in 1.5-2.1-fold higher Cd sequestration in the CS-amended root than control. As a direct result, the root-to-shoots Cd translocation was reduced significantly by 42-51%, while a slightly less significant decrease in brown rice Cd was obtained with the CS treatment relative to control. The CS amendment showed differing effects on brown rice mineral accumulation, with 1.2-1.5-fold increase in brown rice Zn and simultaneously reduced Fe, Mn, Mg and Cu in brown rice. Our results call the readers' attention to the potential impact of soil ameliorator on grain mineral uptake, and we suggest that proper fortification with mineral fertilizers should be supplemented to assist sustainable rice production with improved mineral nutrition.

Influence of metal-contamination on distribution in subcellular fractions of the earthworm (Metaphire californica) from Hunan Province, China.

Earthworms have the ability to accumulate of heavy metals, however, there was few studies that addressed the metals in earthworm at subcellular levels in fields. The distributions of metals (Cd, Cu, Zn, and Pb) in subcellular fractions (cytosol, debris, and granules) of earthworm Metaphire californica were investigated. The relationship between soil metals and earthworms were analyzed to explain its high plasticity to inhabit in situ contaminated soil of Hunan Province, south China. The concentration of Cd in subcellular compartments showed the same pattern as Cu in the order of cytosol > debris > granules. The distribution of Zn and Pb in earthworms indicated a similar propensity for different subcellular fractions that ranked as granules > debris > cytosol for Zn, and granules > cytosol > debris for Pb. The internal metal concentrations in earthworms increased with the soil metals (p<0.05). Significant positive correlations were found between soil Cd and Cd concentrations in cytosol and debris (p<0.01). Moreover, the soil Pb concentration significantly influenced the Pb concentrations in cytosol and debris (p<0.01), similar to that of Cd. The soil Cu concentrations was only associated with the Cu in granules (p<0.05). Soil Zn concentrations correlated with the Zn concentrations in each subcellular fraction (p<0.05). Our results provide insights into the variations of metals partitioning in earthworms at subcellular levels and the relationships of soil metals, which could be one of the detoxification strategies to adapt the long-term contaminated environment.

Effectiveness and potential risk of CaO application in Cd-contaminated paddy soil.

In this study, the accurate effectiveness of CaO in mitigating Cd bioavailability in paddy soil-rice system was investigated and moreover, the potential for reversibility of CaO liming process was provided. Increasing soil pH to ≥6.5 by CaO was determined to be the minimal threshold for minimizing Cd transfer into rice in historically contaminated soils across light to severe Cd levels, while an elevated CaO ratio was needed for soil with recent input of Cd. In CaO remediation treatment, a reduced pool of bioavailable Cd in rhizosphere soil coupled with an increased Cd retention by Fe plaque and an inhibited planta Cd transfer was determined consistently and proposed to be largely responsible for the significant reduction in brown rice Cd. Under continuous simulated acid precipitation test, a negligible level of Cd in eluate with ∼1 unit higher pH than control was recorded with CaO. Significant acidification, however, increased Cd solubilization in the limed soil than in unlimited control, notably at pH 5.5-6.5. As indicated by Visual MINTEQ, a higher Cd-carbonate solubility and much decreased Cd complexation on Fe-(hydr)-oxides across this pH range occurred as a result of elevated Ca input. This suggests that a high soil pH (≥6.5) needs to be permanently maintained once liming has been performed to minimize Cd mobilization. Therefore, amending Cd-contaminated soils with CaO is a cost-effective remedial measure for reducing Cd bioavailability to paddy rice, while a high soil pH need to be permanently maintained to sustain this beneficial effect.